Polymers of unsaturated carboxylic acids and salts thereof are well known. These polymers include homopolymers and copolymers which contain up to 10 weight percent of other copolymerizable monomers. Typical monomers include acrylic acid, methacrylic acid, maleic acid or its anhydride, itaconic acid, and the like. U.S. Pat. No. 2,798,053, for instance, discloses copolymers of acrylic acid with small amounts of polyalkenyl polyether crosslinkers which are gel-like and, especially in the form of their salts, can absorb large quantities of water or solvents with subsequent substantial increase in volume. U.S. Pat. Nos. 3,940,351 and 4,062,817 describe polymers of an unsaturated carboxylic acid and at least one acrylic or methacrylic ester wherein the alkyl groups contain 1 to 30 carbon atoms. Such polymers are also effective thickening agents, even in the presence of substantial amounts of inorganic salts. U.S. Pat. Nos. 3,915,921 and 4,066,583 disclose preparation of same or similar polymers in similar systems.
U.S. Pat. No. 4,267,103 discloses polymers of unsaturated carboxylic acids or salts thereof in certain solvents wherein more than 1% by weight of the carboxyl groups are neutralized. Such polymers have molecular weight greater than 500 and up to several million, but generally, in the range of 10,000 to one million. Such polymers are also effective thickening agents.
U.S. Pat. No. 4,758,641 discloses polymerization of acrylic acid in a solvent selected from acetone, alkyl acetates, and mixtures thereof. Polymerization is carried out in the presence of an effective amount of an initiator and crosslinker and some of the carboxyl groups on the acrylic acid are neutralized.
Polycarbophil is defined as polyacrylic acid crosslinked with divinyl glycol. Polycarbophil, therefore, is a free acid polymer which has numerous applications due to its capacity to absorb fluids, such as water, and its bioadhesive property of being able to adhere to a mucous membrane in the eyes, nose, mouth, gastrointestinal tract, vaginal cavity and rectal canal. A salt of Polycarbophil, such as calcium Polycarbophil, does not have the bioadhesive property and is not used in applications where the material would be expected to attach itself.
In the past, Polycarbophil was made by polymerizing acrylic acid monomer in a concentrated aqueous salt solution, such as magnesium sulfate, or another water soluble nonredox multivalent inorganic salt, in the presence of an initiator and the divinyl glycol crosslinker. The resulting polymer was washed with water several times whereupon it swelled to many times its volume, to remove the salt therefrom. After washing with water, the swollen polymer was dried until a solid chunk of the polymer was obtained, which was only a fraction of its swollen size. The dried polymer was then ground to the desired particle size and used in applications where its swelling property and/or its bioadhesive property were needed.
To reduce the drying cost, the polymer, after its formation, was treated with calcium carbonate or calcium hydroxide whereby the swollen polymer was collapsed upon formation of the calcium Polycarbophil. The calcium Polycarbophil was dried to a solid chunk and then ground to the desired particle size. Whereas Polycarbophil has swelling and bioadhesive properties that make it difficult to pass through the wet mouth, calcium Polycarbophil is ideally suited for oral dose form as it does not swell or become slimey in the mouth. The free acid polymer is liberated in the acid stomach from the calcium Polycarbophil.
Due to the extended and costly manufacturing procedure, only the salt of Polycarbophil is generally available.